The present invention relates to a method for determining the geometric position of the rotational axis and the center of rotation of a vehicle wheel in a measurement station coordinate system, while the motor vehicle is being driven, by using at least two optical recording devices assigned to each other in terms of position, and by using a downstream evaluation unit that processes the image information that was captured, with consideration for several wheel features present on the wheel or attached for measurement purposes, and by using at least one body feature present on the body or attached for measurement purposes. In the method, 2D coordinates of the wheel features and the at least one body feature are detected in a time-synchronized manner, and, based thereon, the 3D coordinates of the features are calculated at certain time intervals, and they are back-calculated to a previously established reference time or a related reference position of the vehicle wheel, with consideration for the distance traveled by the at least one body feature relative to the reference position.
A method of this type is described in DE 100 50 653 A1. With this known method, the axis of motion, and data on the wheel geometry and axle geometry are ascertained with the aid of cameras, a reference coordinate system at the measurement station, and optical features on the vehicle wheel and the vehicle body. Similar methods for performing an optical measurement of wheel and axle geometry data are also presented in DE 199 34 864 A1 and DE 197 57 760 A1, in which cases the measurement is also carried out as the motor vehicle is driven past.
The aforementioned methods are based on the measurement of features that are present on the vehicle or that are attached specifically for measurement purposes. In addition to wheel features, at least one body feature and a reference feature system for referencing the measuring device are also provided. To determine the plane of wheel rotation, time-synchronized images are taken continually at different wheel positions (based on the angle of rotation) and different vehicle positions using the measuring devices. Based on the chronologically successive coordinates of the body features, the trajectory of the motor vehicle is determined relative to the reference features. The plane of wheel rotation is determined based on the positions of the coordinates of the wheel features captured in the various rotational positions of the wheel, with consideration for the vehicle motion. It is possible to ascertain the amount of a form/shape error of the wheel rim (rim runout), and to take it into account. Using this information, it is now possible to calculate the vehicle coordinate system and the data on the wheel and axle geometry. A description of the method for determining the plane of wheel rotation is not disclosed in detail.
Previously, roller sets or counterrotating slide plates were used to rotate the wheels while the vehicle was at a standstill in order to determine the rim runout as a correcting quantity for the axial measurement.
Methods are also known with which, in three different rolling positions of the wheel on the very flat surface of a wheel alignment station, the angular positions of angle sensors mounted on the wheel or of optical targets mounted on the wheel—the optical targets being composed of a precise configuration of individual, optically detectable surfaces—are detected by the wheel alignment station and, based thereon, a rim runout compensation is calculated. Targets of this type are described in detail, e.g., in U.S. Pat. No. 6,252,973.
The determination of rim runout while the wheel is lifted is no longer permitted by most vehicle manufacturers today, because the strain that occurs in the wheel suspension after the wheels are set down results in considerable errors in the subsequent measurement of wheel alignment.
With a method described in DE 42 12 426 C1, the wheels of the motor vehicle rotate in rolling prisms while the vehicle is at a standstill. A marking is placed on the wheels outside of the axis of rotation that may be registered optically and that is detected using two synchronized cameras while the wheel is rotating. The axis of rotation, toe and camber are determined based on the spacial positions of the markings on the wheel.
The object of the present invention is to provide a method for determining the axis of rotation and the center of rotation of a vehicle wheel, with which measured results that are as reliable and exact as possible are obtained during travel, in particular on a real road.